1. Field of the Invention
This invention relates to an improved method and apparatus for producing from thermoplastic polymers flat slabs or billets of substantial weight and thickness, free of internal voids and other defects, suitable for conversion to useful articles without remelting.
2. Description of the Prior Art
In recent years a significant industry has developed for the production of thick-sectioned slab stock for use in fabricated structures like tanks, machinery bases and machined parts such as gears, wheels, and bearings. Another factor generating increasing interest in these slabs is the development of "solid phase forming" processes. These techniques involve the shaping of billets, cut from sheet or slab stock. At a temperature just below the crystal melting point the billets can be forged, stamped, or deep drawn into a wide variety of parts. The molding or thick-sectioned thermoplastic slabs or parts, e.g., about 0.5-5 cm (or about 0.25 in. to 2 in.) thick, is inherently difficult and time consuming. The time required for solidification is long because of the low thermal conductivity of all plastics. This time increases approximately as the square of the thickness. The second factor of importance is the large volumetric shrinkage of thermoplastics when cooling from a melt to a room temperature solid. This is especially significant in the case of crystalline polymers like polypropylene and high density polyethylene (HDPE) which shrink about 15 and 25 volume percent, respectively, during the cooling cycle. If one attempts to injection mold thick parts, this volume change shows up as shrinkage away from the mold surfaces and/or voids within the part. In addition to the creation of sinks and voids, the shrinkage away from the mold further reduces the rate of cooling and extends the cycle.
The methods routinely used to produce thick plastic slabs are sheet extrusion and compression molding. Sheet extrusion of thick stock is often slow and the product requires stress relieving for many fabricated applications. Compression molding usually involves loading a large mold with pellets, melting the pellets with heated platens, then cooling the platens and solidifying the product. This compression molding is usually done under high pressures to minimize void formation and sinking. Pressures of about 35-70 bars (about 500-1000 psi) are common, requiring heavy and expensive press equipment.
The prior conventional procedure of pressing pellets against heated platens to prepare a thick slab is time-consuming and inefficient. In searching for a more economical method, I first concluded that the melting function should be carried out in an extruder. Consequently, I devised the method described in my U.S. Pat. Nos. 3,608,058 and 3,733,159, which comprises extruding a thick "blob" of melt into a mold in which the side-wall defining member is made a deformable material, such as a rubber ring, and the billet is formed by compressing the ring mold between cooled flat platens.